The present invention relates generally to a voice coil motor (abbreviated as VCM) and, more particularly, to a voice coil motor provided with a set of improved shorted turn.
Typically, a shorted turn is disposed in a voice coil motor which is conventionally incorporated into a magnetic disk drive or an optical disk drive. It is desired that the inductance of a coil in the voice coil motor will decrease as a result of the incorporation of the shorted turn so as to decrease the access time of a magnetic head in the disk drive.
As shown in FIGS. 1, 2, 7 and 8 (all prior art), a conventional voice coil motor 10 includes an upper sectoral yoke 12, a lower sectoral yoke 14, a central arcuate yoke 16, two upper side yokes 18 and 20, two lower side yokes 22 and 24, an upper magnet 26, a lower magnet 28, and a rotary arm 30. The two upper side yokes 18 and 20 are respectively disposed at laterally opposite end positions on a lower surface of the upper yoke 12. The two lower side yokes 18 and 20 are respectively disposed at laterally opposite end positions on an upper surface of the lower yoke 14. The two distal end portions of the central yoke 16 are respectively sandwiched between the two upper side yokes 18 and 20 and the two lower side yokes 22 and 24. Bolts or screws are used to fasten the upper and lower sectoral yokes 12 and 14, the upper and lower side yokes 18, 20, 22, 24 and the central yoke 16 together (as shown in FIG. 8 (prior art) ). The rotary arm 30 is pivotally mounted on the upper yoke 12 and has a through hole 32 at a tail end thereof. A coil 34 is wound along the inside wall of the through hole 32, and is shown removed from the through hole 32 in FIG. 2. (prior art). The tail end of the rotary arm 30 is mounted to the central yoke 16 with the yoke 16 extending through the through hole 32. A spacing is maintained between the central yoke 16 and the rotary arm 30 so that the coil 34 will not interfere with the yoke 16 when the rotary arm 30 is pivoted. When incorporated in a magnetic disk drive or an optical disk drive 5 as shown in FIG. 1 (prior art), the rotary arm 30 is provided with several magnetic heads 36 at a leading end thereof. A conventional shorted turn 40 is arranged to surround the central yoke 16 and is thus in the form of a hollow arcuate tube having a rectangular cross section which corresponds to the cross section of the central yoke 16 (FIGS. 7 and 8 (both prior art)).
For a magnetic disk drive, the access operation which a read/write head executes on a disk is driven by a voice coil motor. As mentioned above, the purpose of providing the shorted turn 40 is to reduce the resultant inductance of this moving coil, and then shorten the access time. The decrease in coil inductance can decrease the time constant (inductance/resistance) directly and increase its response sensitivity. However, there are other factors such as the magnitude of output of VCM, which is determined by field magnetic flux density and coil length. The minimum access time can not be attained merely by decreasing the inductance of coil, nor by improving any factor unilaterally.
From an article, "The Actuator in High Performance Disk Drives: Design Rules for Minimum Access Time", J. Arthur Wagner, IEEE Transactions on Magnetics, Vol. MAG-19 , No. 5, Sept. 1983, it may be understood that the parameters which affect the access time of a voice coil motor include:
1) moment inertia (mass if a linear movement);
2) torque constant ( K.sub.T =BLr ) (K.sub.T =BL if a linear movement);
3) voltage applied;
4) resistance of coil; and
5) inductance of coil (including shorted turn).
If values for the torque constant and the inductance are changed by 10% respectively, the access time will be changed by 4 to 5% and 0 to 1.5% respectively. Therefore, it may be understood that the torque constant value is more sensitive to the access time than the inductance. That is, it is more effective to increase the torque constant value than to decrease the inductance value. For the same coil, the magnitude of the magnetic flux density is more important than the inductance.
The conventional shorted turn identified above is mounted on the central yoke. Although it is said that the conventional shorted turn may decrease the inductance to a minimum value, it has been noted that the thickness of shorted turn causes an increase in the spacings between the magnets 26, 28 and the central yoke 16. Inevitably, each magnetic flux density decreases as the spacing increases. It is therefore a disadvantageous effect to the access time. Furthermore, the conventional shorted turn 40 is unmovably fitted around the central yoke and difficult to manufacture and assemble. The invention should also assure that when the rotary arm 30 pivots and thus moves, the coil 34 along the shorted turn 40, it the coil 34 does not interfere with the shorted turn 40. Since the spacing between coil 34 and shorted turn 40 is intended to be kept small, the accuracy for machining a conventional shorted turn is required to be high. If a voice coil motor for driving a linear movement and having a shorted turn in the form of a hollow, rectangular-section sleeve is manufactured, a precise wire discharge cutting machine is needed. Instead, if a voice coil motor for driving a rotary movement and having a shorted turn in the form of an above-mentioned arcuate, hollow, rectangular-section sleeve is manufactured, still more difficulties encountered plus a higher cost is incurred.
In the field of manufacture of the voice coil motor, the limit to the use of materials has been reached. That is to say, the access time of magnetic disk drives on the same product design order has reached its limit.